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Bureau of Mines Information Circular/1988 




Probe-Hole Drilling: High-Stress 
Detection in Coal 



By John P. McDonnell and Khamis Y. Haramy 




UNITED STATES DEPARTMENT OF THE INTERIOR 




Information Circular 9179 



Probe-Hole Drilling: High-Stress 
Detection in Coal 



By John P. McDonnell and Khamis Y. Haramy 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
T S Ary, Director 



Tltas 



Library of Congress Cataloging in Publication Data: 



McDonnell, John P. 

Probe-hole drilling. 

(Information circular/United States Department of the Interior, Bureau of Mines ; 9 1 79) 

Bibliography: p. 11. 

1. Rock bursts. 2. Coal mines and mining — Safety measures. 3. Boring. I. Haramy, Khamis 
Y. II. Title, in. Series: Information circular (United States. Bureau of Mines) ; 9179. 



TN295.U4 [TN317] 622 s [622 '.8] 88-600003 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Probe-hole drilling 2 

Theory 2 

Background 3 

Case study 3 

Mine 3 

Geology 3 

General background 3 

In-mine probe-hole drilling. . . 5 

Procedure 5 

Results 5 

Laboratory tests 7 

Procedure 8 

Results 9 

Conclusions 10 

References 11 

ILLUSTRATIONS 

1. Boundary stress concentration for circular opening 2 

2. General layout of the mine and drilling test area 4 

3. Sample field data form 6 

4. Drilling yield field results 7 

5. Longwall face area showing probe-hole locations 8 

6. Laboratory equipment setup for drilling yield tests 9 

7. Relationship between applied load and cuttings volume 10 

8. Drilling yield laboratory results 10 

TABLES 

1. Coalcrete physical property data 9 

2. Drilling yield laboratory test data 10 





UNIT OF MEASURE ABBREVIATIONS USED 


IN 


THIS REPORT 


ft 


foot in 






cubic inch 


gal 


gallon lb 






pound 


gal/ft 


gallon per foot mL 






milliliter 


h 


hour psi 






pound per square inch 


in 


inch vol 


P< 


2t 


volume percent 



PROBE-HOLE DRILLING: HIGH-STRESS DETECTION IN COAL 

By John P. McDonnell 1 and Khamis Y. Haramy 1 



ABSTRACT 

Coal mine bounces and bursts are major problems facing U.S. mine oper- 
ators. Bounces and bursts have the potential to inflict severe injury 
on mining personnel, damage equipment, and cause mine closures. High- 
stress conditions, at or near the working face, are the common denomina- 
tor in the burst problem. If mine operators can locate high-stress and 
potentially burst-prone zones, they can then use stress-relief methods 
to control the burst condition. 

One method of locating the high-stress zone is the probe-hole-drilling 
or drilling-yield method. Probe-hole drilling is used frequently in 
Europe, the U.S.S.R., and Japan as a means for locating potential burst 
zones. Consequently, the Bureau of Mines performed tests in the labo- 
ratory and in a deep, burst-prone western mine to analyze probe-hole 
drilling. The in-mine method, very simply, involves drilling a hole 
into the coal seam and measuring the volume of cuttings obtained. A 
certain volume of cuttings can be expected from a certain diameter and 
length drill hole. A significant increase in the volume of cuttings 
means the zone around that particular hole is highly stressed. In-mine 
use of the drilling yield method has shown it to be a useful tool for 
locating highly stressed and potential burst zones. 

Results from laboratory testing confirm that high stress applied tri- 
axially to a cube specimen will cause a significant increase in the vol- 
ume of cuttings from a small-diameter drill hole in the specimen. 



'Mining engineer, Bureau of Mines, Denver Research Center, Denver, CO. 



INTRODUCTION 



Bounces and bursts have become a common 
occurrence in U.S. coal mines. Between 
1978 and 1984 there were 73 accidents 
attributed to bounces and/or bursts. As 
coal mine operators attempt to mine deep- 
er reserves, the problems could increase. 

Common to the burst problem is the oc- 
currence of high-stress zones in the area 
of the active face. Such high-stress and 
potential burst zones can be attrib- 
uted to many factors; much research has 
been conducted in attempts to precisely 
determine causes of bounces and bursts. 
High-stress zones, regardless of their 
causes, significantly contribute to burst 
occurrences. Locating the high-stress 
zones in the working face is important to 
eventual burst control. When the high- 
stress zones are located, they can be de- 
stressed. As a result, the burst poten- 
tial can be controlled. 



This paper describes the analysis of 
the probe-hole-drilling or drilling-yield 
method, which has been used in several 
countries as a practical tool to detect 
high-stress zones at the working face in 
underground coal mines. The method in- 
volves drilling a certain diameter and 
length hole into the coal, measuring the 
volume of cuttings obtained, and compar- 
ing the volume of cuttings to the volume 
of the hole. Laboratory tests were con- 
ducted to determine if there is a cor- 
relation between the magnitude of the 
stress and the volume of cuttings ob- 
tained. A coal mine in western Colorado, 
which regularly experiences high-stress 
conditions, was used as a field site 
for probe-hole drilling. This paper de- 
scribes the laboratory and in-mine probe- 
hole-drilling results. 



PROBE-HOLE DRILLING 



THEORY 

The theory behind probe-hole drilling 
comes from the concept of stress around a 
circular opening. The magnitude and dis- 
tribution of the stresses around a sin- 
gle underground opening in massive rock, 
e.g. , a drill hole in a thick coal seam, 
have been determined analytically and 
from laboratory model studies (_1_). The 
stress concentrations around a circular 
opening in a bidirectional stress field 
are shown in figure 1. This figure shows 
that when Poisson's ratio for a material 
equals 0.25, the boundary stress concen- 
trations around the circular opening can 
be obtained. For example, if the applied 
stress, Sv, equals 1,000 psi, then the 
boundary stress at point A on the circu- 
lar opening equals 1,000 X 2.6, or 2,600 
psi. 

. 

^Bounce, as used in this paper, refers 
to a sudden, forceful impact or jolt, 
which may be accompanied by face or rib 
sloughage; a burst involves an explosive 
release of material. 

•^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of the report. 



When the boundary stress exceeds the 
strength of the material, the hole begins 
to deform and fail. When the applied 
stress is high to begin with, as is the 
case with forward abutments ahead of 
longwall faces, the coal around the hole 
fails and behaves plastically. The coal 
around the drill hole in the plastic zone 
flows into the hole and results in large 
volumes of cuttings. 



a> 



3 






































1. 
"3 


















M 






















1 






A 


/ 








































































1 

— 


1 1 

S 
11 


1 
1 

) 

t 


1 


/ 
/ 

\ 


1 

1 
/ 

3* 


/ 
/ 

/ 

f 

* 


/ 
/ 
/ 


/ 
/ 
/ 



KEY 
M Ratio of S h /S v 
S v Vertical stress 
S n Horizontal stress 

°9 Stress at angle 9 



FIGURE 1.— Boundary stress concentration for circular 
opening. 



BACKGROUND 



2. The auger steel becomes stuck. 



Probe-hole drilling has been used in 
Germany since the 1960's, in the U.S.S.R. 
since the 1950's, in Poland since 1965, 
and in Yugoslavia since 1968 (2^3) • i n 
the United States, Talman and Shroder (4_) 
reported experiences with large-diameter, 
6-in, auger-drilled holes that were used 
as a stress-relief method in the 1950's. 
The large-diameter holes actually trig- 
gered coal bursts. 

The in-mine drilling-yield method in- 
volves drilling small-diameter holes into 
the coal panel or pillar, usually with an 
auger drill, and recording certain infor- 
mation while drilling, such as the volume 
of drill cuttings obtained from certain 
depths in the hole. A certain volume of 
cuttings can be expected from a drill 
hole of a particular diameter and length. 
If the actual volume of cuttings gener- 
ated exceeds the volume of the hole by 
a significant amount, the seam in that 
location is determined to be highly 
stressed and potentially burst prone. 

The high-stress zone is determined by 
the following criteria: 

1. The ratio of volume of drill cut- 
tings obtained to the volume of the hole. 
If the ratio greatly exceeds 1, the zone 
is potentially burst prone. 



3. The probe-hole drilling triggers a 
bump (noise) or a bounce or burst. 

The high-stress-zone criteria can in- 
clude zones of gas, cuttings size, and 
other driller experiences; for example, 
the auger steel being drawn into the 
hole, similar to the action of a wood 
screw, indicates the intersection of the 
high-stress zone. Any combination of the 
criteria can indicate a potential burst 
zone. The experience of the drillers at 
each operation will dictate what criteria 
work for that operation or that partic- 
ular site. Well-disciplined probe-hole 
drillers try to prevent triggering bursts 
by taking precautions such as selecting 
proper auger size, using proper drilling 
techniques, and paying close attention to 
the actions of the hole being drilled. 

Probe-hole drilling does not give the 
absolute stress magnitude of the coal 
seam. The purpose of the laboratory 
tests was to confirm the theory that high 
stress will produce large volumes of 
drill cuttings. The absolute magnitude 
of the stress is unimportant for this 
method. What is critical is the ability 
to locate the high-stress zone before 
mining into it. 



CASE STUDY 



MINE 



The case study mine is located in west- 
ern Colorado. The mine workings are un- 
der approximately 3,000 ft of overburden. 
The mine utilizes the advancing longwall 
method. Figure 2 shows the general lay- 
out of the mine and the test area. The 
coal seam dips to the southeast at ap- 
proximately 12° with the longwall panels 
oriented fairly parallel to the strike of 
the seam. 

GEOLOGY 

The coal seam, approximately 10 ft 
thick, is surrounded by hard roof and 
floor material. The immediate roof, ap- 
proximately 5 ft thick, is composed main- 
ly of strong siltstone, shales, and 
sandstone layers. The immediate roof is 



overlain by a 9-ft competent sandstone 
layer that does not readily fracture. 
The mine floor consists of a strong 
shale-sandstone layer 4 to 10 ft thick, 
and a lower coalbed beneath the shale- 
sandstone floor which consistently mea- 
sures 8 to 10 ft thick. The compressive 
strength of the coal is 2,440 psi, while 
Young's modulus is 5.2 x 10 psi. 

GENERAL BACKGROUND 

Owing to the thick overburden and the 
existence of strong roof and floor mate- 
rial, bounces and bursts have occurred in 
this mine in both room-and-pillar and 
longwall sections. Longwall panel A ex- 
perienced six significant bursts as the 
face advanced to 620 ft from the starting 
room of the panel. 




Scale, ft 



Room-and-pillar gob areas 
Longwall gob areas 
Fault 

Probe-hole— dri Ming test area 



FIGURE 2.— General layout of the mine and drilling test area. 



After a burst, when the face was at 
620 ft, mine management decided to incor- 
porate a burst-control plan into the 
daily operations. The program consisted 
of detecting high-stress areas in the 
coal face using probe-hole drilling and 
then destressing high-stress areas using 
volley firing. Volley firing consists 
of fracturing certain zones of the face 



using explosives. Where the face is 
blasted and fractured, no stress can 
build up; the face is destressed. After 
the stress detection program was incorpo- 
rated into the mining cycle, the longwall 
panel was mined to completion, nearly 
3,900 ft, with essentially no uncon- 
trolled burst occurrences. 



IN-MINE PROBE-HOLE DRILLING 



PROCEDURE 

Probe-hole drilling at the study mine 
was used to locate high-stress zones, 
which were then destressed. Drilling op- 
erations utilized a Turmag 4 model hand- 
held, air-powered auger drill (5_~6) • 
Auger flights, which permitted either air 
or water flushing of the hole, were gen- 
erally 3 to 5 ft long. Along with the 
auger flights, a two-wing, 2-in-diam, 
carbide-insert drag bit was used to cut 
and clean the hole. Drilling operations 
consisted of a two-man crew, one driller 
and one helper; the latter assisted in 
adding auger flights, measuring drill 
cuttings, and recording drilling informa- 
tion. Site preparation involved scaling 
the rib or face to provide a solid, sta- 
ble surface for the collar of the hole. 
Site preparation generally took as long 
as or longer than actual drilling. Ac- 
tual drilling operations involved con- 
trolling penetration rate and adding 
auger flights. Drilling operations were 
performed while the longwall face was not 
operating. This permitted the driller 
to listen to the activity caused by the 
drilling. 

Because the drill is hand-held, the 
driller's experiences are critical to 
locating the high-stress zones. The 
driller actually "feels" the hole squeez- 
ing around the auger flights. The 
driller controls the penetration rate to 
prevent sticking the auger steel in the 
hole. Data collection includes recording 
the volume of cuttings generated per 
given length of drill hole, cuttings 

Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



size, presence of gas or water, hole 
squeeze, occurrence of bounces, etc. A 
sample field data form is shown in figure 
3. 

The following criteria for locating the 
high-stress zone at the study mine were 
used: 

1. Cuttings (Vc) were in excess of 5 
gal per 3 ft of hole length. The ex- 
pected volume of cuttings (Ve) from a 
3-ft-long, 2-in-diam hole is 0.5 gal. 
Therefore, where the ratio of actual cut- 
tings (Vc) to expected cuttings (Ve) was 
greater than or equal to 10, a high- 
stress zone was determined. 

2. Auger became stuck. 

3. Drilling generated a bump, bounce, 
or burst. 

The presence of gas blowing from the hole 
was also used as an indicator of a high- 
stress zone. 

RESULTS 

The drilling-yield program at the study 
mine consisted of drilling probe holes 
on 50- to 100-ft centers along the length 
of the longwall face and recording the 
drilling information. Probe-hole drill- 
ing was performed on a daily basis. 

A plot of the drilling-yield results 
shows the location of the stress abutment 
peak ahead of the face. Figure 4 shows 
typical drilling-yield data from three 
drill holes. In hole 1, for example, 
there is more than one small abutment 
zone within the length of the drill hole 
at this location. The maximum abutment 
zone for hole 1 occurred at a depth of 



Date: 

Hole diameter: 



Shift: DrillerL 
Hole location: 



DRILLING RECORD 

Drilling start: 

Drilling end: 



h 
h 



+ Low noise 

O Noise can be heard 60 ftaway 

• Noise can be heard 300 ft away 



Depth ... ft 


3 


6 


9 


12 


15 


18 


21 


24 


27 


30 


Volume of 
cuttings, gal 






















+ 






















O 






















• 






















Remarks 






















Depth., .ft 


33 


36 


39 


42 


45 


48 


51 


54 


57 


60 


Volume of 
cuttings, gal 






















+ 






















O 






















• 






















Remarks 























REMARKS, CODES 
B-Gas is blowing out of hole W— Drill cuttings are damp 
F- Drill cuttings are fine Z —Drill is drawn into the coal 

G-Drill cuttings are coarse ST- Rock ahead 
K-Coal pressure squeezes drill WW-Drill cuttings are wet 



FIGURE 3.— Sample field data form. 



o 

LU 

_J 

§ 

CO 



h- o- 



u 








10 15 



20 25 30 35 40 45 
DEPTH INTO HOLE, ft 

FIGURE 4.— Drilling yield field results. 



50 55 60 



53 ft. The distance from the face to the 
abutment zone varies from hole to hole. 
In hole 3, for example, the abutment zone 
is at a depth of only 39 ft. 

Figure 5 shows the longwall face area 
with other probe-hole locations, high- 
stress zones, and blowing-gas locations. 
This is a typical plot of the probe-hole 
information. On this sketch are the lo- 
cation and hole depth of the high-stress 
zone and the blowing-gas line (5_ ) . No- 
tice that the gas line nearly parallels 
the stress abutment line. 

At this mine, if the drilling-yield 
results showed the abutment zone to be 



approximately three times the seam height 
away from the face, the face was gener- 
ally said to be non-burst-prone. Com- 
bined with driller and miner experiences, 
the "three times" rule provided fairly 
good information regarding locations of 
high-stress areas and whether the face 
could be safely mined. Operators at 
other mines experiencing bounce and/or 
burst conditions need to determine high- 
stress criteria that 
particular operations, 
scribed here can be 
guidelines. 



apply to their 
The methods de- 
used as general 



LABORATORY TESTS 



Probe-hole drilling, as proven by the 
in-mine testing, can give reliable infor- 
mation on the general state of stress in 
the coal seam. The absolute magnitude of 
the stress is not determined, however. 
Also, the orientation of the stress is 



not and cannot be determined by probe- 
hole drilling. 

The laboratory tests were performed to 
quantify, to some degree, what stress 
magnitude would produce an excessive vol- 
ume of cuttings. 



Panel 



^723) 
\ Stress/ t" 



t. •'<£ jAJ.-LT. 1 J^ . o .<=>•• A-> -.w. ■ -y , 



o. " 



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^assess 



liiillil 









®?imm>mm 










5 



Probe-hole depth to stress, ft 

Hole clear of stress and 
gas to 50 ft 

Probe-hole depth to gas, ft 



FIGURE 5.— Longwall face area showing probe-hole locations. 



PROCEDURE 

Laboratory tests were conducted on 
cubes of coalcrete, a mixture of coal, 
fly ash, cement, and water in the follow- 
ing proportions: 

Material Vol pet 

Coal 47 

Fly ash 37 

Cement , 7 

Water 9 

Fly ash, a product of burned coal, var- 
ies depending on the specific coal type 
and coal properties. A mix containing 
equal amounts of class C and class F 
fly ash was selected for use in the 



tests; class C contains higher concentra- 
tions of alumina and cementlike materials 
than class F. 

Physical property tests on the coal- 
crete mixture were performed in the Bu- 
reau of Mines rock testing laboratory. 
Physical properties of the coalcrete are 
listed in table 1. The coalcrete exhib- 
its similar material properties to those 
of weak coal. 

Laboratory tests were conducted on 4-in 
coalcrete cubes. Figure 6 shows the com- 
plete equipment setup, which included a 
120,000-lb-capacity hydraulic press, a 
steel test frame, steel flat jacks filled 
with hydraulic fluid, and a hand-held 
variable-speed drill with a 9/16-in drill 
bit. 




FIGURE 6.— Laboratory equipment setup for drilling yield tests. 



TABLE 1. - Coalcrete physical 
property data 

Number of samples 9 

Sample dimensions, in: 

Length 4.23 

Diameter 2.09 

Lateral pressure psi.. 

Compressive strength, psi: 

Average 1,430 

Range.... 1,122-2,201 

Young's modulus, 10 6 psi: 

Average 0.35 

Range 0. 28-0.48 

Poisson's ratio: 

Average 0.28 

Range 0. 17-0.42 

The steel test frame was designed with 
the steel flat jacks to provide a means 
for applying variable lateral confining 
pressures, while the hydraulic press was 
used to control the vertical applied 
load. The test procedure involved con- 
fining the cube in the test frame, apply- 
ing lateral and axial loads, drilling the 
cubes, collecting the cuttings, and mea- 
suring the volume of cuttings. 



RESULTS 

The laboratory tests were conducted as 
follows : 

1. The 4-in cube of coalcrete was 
placed in the test frame with confinement 
applied laterally. The lateral confine- 
ment was only 300 to 500 psi. Higher 
confining pressures were used, but this 
limited the ability of the coalcrete to 
fail when drilled. 

2. A 9/16-in-diam hole was drilled 
through the width of the cube. The cut- 
tings were collected and measured. This 
value was used as the expected volume of 
cuttings from an unstressed cube, Ve. 

3. Vertical stress was applied to the 
cube by the press in increments of 500 to 
1,500 psi. 

4. At each incremental load level, the 
cube was redrilled, and additional 
cuttings were collected. At low stress 
levels, the additional volume of cuttings 



10 



was very small. As the stress increased, 
so did the volume of cuttings until fi- 
nally the cube failed, and the cuttings 
flowed from the hole while drilling con- 
tinued. The cumulative volume of cut- 
tings from each individual hole was 
termed Vc. 

Data summarizing the drilling-yield 
tests are listed in table 2. The ratio, 
Vc/Ve, was obtained by dividing the ac- 
tual volume of cuttings obtained, Vc, by 
the volume of cuttings obtained while 
drilling the unstressed cube, Ve. Figure 
7 shows the typical relationship between 
the applied vertical stress and the vol- 
ume of cuttings; as the stress increased, 
so did the volume of cuttings, until 
finally the cube failed. The lateral 
stress was maintained at 300 to 500 psi. 
Nineteen samples were tested. The gen- 
eral plot of figure 7 was consistent from 
cube to cube. 

Figure 8 shows the accumulation of the 
data for each incremental load value. 
For coalcrete cubes tested in the labora- 
tory and laterally confined by only 300 
to 500 psi, a definite relationship ex- 
ists between the applied vertical stress 
and the ratio Vc/Ve. As the stress in- 
creased, so did the volume of drill 
cuttings. 

TABLE 2. - Drilling yield 
laboratory test data 



Applied 

stress, 

Psi 

1,630.. 
2,220.. 
3,270.. 
4,440.. 
4,900.. 
6,530.. 



Average 
Vc/Ve 

1.0 
1.0 
1.1 
1.2 
1.4 
1.9 



Applied 

stress , 

psi 

6,660... 

8,160... 

8,880... 
11,110... 
12,220... 
12,780... 



Average 
Vc/Ve 

1.6 
2.4 
2.4 
2.7 
3.2 
3.5 




10 20 30 

CUMULATIVE VOLUME OF CUTTINGS, mL 



40 



FIGURE 7.— Relationship between applied load and cuttings 
volume. 




0.1 0.2 0.3 0.4 0.5 0.6 

Log Vc/Ve 
FIGURE 8.— Drilling yield laboratory results. 



CONCLUSIONS 



Probe-hole drilling, or drilling yield, 
is a practical method for locating high- 
stress and potentially burst-prone zones 
in the coal face. Although the method 
has been used in foreign countries since 
the 1950's, it has seen only limited use 



in the United States. As coal mine oper- 
ators mine deeper reserves and experience 
more burst conditions, this method of 
high-stress detection could become more 
widely used. 



11 



Probe-hole drilling, very simply, in- 
volves drilling a small-diameter hole 
into the coal seam and collecting in- 
formation while drilling, such as the 
volume of cuttings, behavior of the drill 
steel, bounces induced by drilling, gas 
blowing from the hole, etc. Depending 
on a combination of the aforementioned 
factors, an area of the face may be de- 
termined to be highly stressed. In mines 
with burst problems, the high-stress area 
is usually burst-prone. When the high- 
stress area has been located and deter- 
mined to be burst-prone, the area can be 
destressed. 



In-mine use of probe-hole drilling 
showed that the method can give the mine 
operator some idea of the relative stress 
level in the coal and, most importantly, 
can provide the location of extremely 
high-stress zones in the active working 
face. The method does not give a precise 
quantitative magnitude of the in situ 
stress, however. 

Tests on laboratory samples proved that 
there is a direct correlation between 
increased stress and cuttings volume. 
The laboratory work confirmed that probe- 
hole drilling can be used to locate high- 
stress zones in the coal seam. 



REFERENCES 



1. Obert, L. , W. Duvall, and R. Mer- 
rill. Design of Underground Openings in 
Competent Rock. BuMines B 587, 1960, 
36 pp. 

2. Coeuillet, R. General Report on 
the Working of Outburst-Prone Mines. Pa- 
per in Proceedings of the Symposium on 
Coal and Gas Outbursts. Nimes , France, 
Nov. 25-27, 1964, pp. 1-23. 

3. Kidybinski, A. Bursting Liability 
Indices of Coal. Int. J. Rock Mech. Min. 
Sci. & Geomech. Abstr. , v. 18, No. 4, 
Aug. 1981, pp. 295-304. 



4. Talman, W. G. , and J. L. Shroder. 
Control of Mountain Bumps in the Poca- 
hontas No. 4 Seam. AIME, v. 211, 1958, 
pp. 888-891. 

5. Varley, F. D. Outburst Control in 
Underground Coal Mines. Paper in Pro- 
ceedings of Fifth Conference on Ground 
Control in Mining. WVU, Morgantown, WV, 
1986, pp. 249-256. 

6. Stewart, C. L. Private communica- 
tion, Sept. 1983; available upon request 
from J. P. McDonnell, BuMines, Denver, 
CO. 



60142 £00 



U.S. GOVERNMENT PRINTING OFFICE: 1988 — 547-000/80,028 



INT.-BU.0F MINES,PGH. ,PA. 28681 



U.S. Department of the Interior 
Bureau of Mines-Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 1S236 



OFFICIAL BUSINESS 
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